HEMT GaN device with a non-uniform lateral two dimensional electron gas profile and method of manufacturing the same

ABSTRACT

A high electron mobility field effect transistor (HEMT) having a substrate, a channel layer on the substrate and a barrier layer on the channel layer includes a stress inducing layer on the barrier layer, the stress inducing layer varying a piezo-electric effect in the barrier layer in a drift region between a gate and a drain, wherein a two dimensional electron gas (2DEG) has a non-uniform lateral distribution in the drift region between the gate and the drain, wherein the stress inducing layer comprises a material having a height that decreases linearly and monotonically in the drift region in the direction from the gate towards the drain, and wherein the 2DEG decreases in density in the drift region between the gate and the drain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 15/005,744 filed on Jan. 25, 2016, which is a divisional ofU.S. patent application Ser. No. 13/478,609 filed on May 23, 2012, andthis application claims the benefit of U.S. patent application Ser. No.15/005,744 filed on Jan. 25, 2016, and U.S. patent application Ser. No.13/478,609 filed on May 23, 2012, which are incorporated herein byreference as though set forth in full. This application is related toU.S. patent application Ser. No. 13/479,018 filed May 23, 2012 and toU.S. patent application Ser. No. 13/478,402 filed May 22, 2012.

STATEMENT REGARDING FEDERAL FUNDING

NONE.

TECHNICAL FIELD

This disclosure relates to type III-nitride high electron mobilitytransistor (HEMT) devices and in particular to two dimensional electrongas (2DEG) in the drift region.

BACKGROUND

A high electron mobility transistor (HEMT) is a field effect transistorincorporating a junction between two materials with different band gaps(i.e., a heterojunction). Gallium nitride (GaN) HEMTs have attractedattention due to their high-power performance. In type III-nitride HEMTdevices used in power applications there is a conflicting designtrade-off between the on-state resistance and breakdown voltage (BV).Since the relation between the BV and on resistance is at leastquadratic, improvement in the BV for a given drift region length resultsin a significant improvement in the FOM of the device, defined asBV²/Ron.

In the prior art type III-nitride HEMT devices have a uniform 2DEGdensity which results in a peak electric field under or near the gateregion. The electric field distribution tends to be closer to atriangular shape than to the desired trapezoidal shape which reduces thebreakdown voltage per unit drift region length of the device. The use offield plate and multistep field plates are some of the techniques thatare used to improve the electric field distribution. However, fieldplates typically result in multiple peaks and suffer from less than anideal flat field distribution, and may exhibit a saw tooth profile.Field plates also add to the gate to drain capacitance. In addition,process complexity and cost typically increase with the number of fieldplate steps.

U.S. Pat. No. 7,038,253 to Furukawa describes a GaN based device onsilicon (Si) technology which uses a uniform 2DEG profile in the driftregion. Because of the absence of any field shaping technique in theFurukawa device, the breakdown voltage and dynamic on resistance fromdrain to source is limited by a localized increase in the electric fieldunder the gate region thus requiring over design of the device whichdegrades the figure of merit (FOM) that such a device can achieve.

In “High Breakdown Voltage AlGaN/GaN HEMTs Achieved by Multiple FieldPlates” by H. Xing et. Al, a field shaping technique that uses multiplefield plates is described to improve the electric field distribution.However, multiple field plates do not achieve a uniform electric field,may have a saw tooth type distribution, and introduce gate to draincapacitance. Implementing such a device structure also increases devicecomplexity and cost.

What is needed is a significant improvement in the FOM in HEMT devices,and in particular an improvement in the breakdown voltage for a givendrift region length, so that the FOM of the device, defined as BV²/Ron,improves. The embodiments of the present disclosure answer these andother needs.

SUMMARY

In a first embodiment disclosed herein, a high electron mobility fieldeffect transistor (HEMT) having a substrate, a channel layer on thesubstrate and a barrier layer on the channel layer comprises a stressinducing layer on the barrier layer, the stress inducing layer varying apiezo-electric effect in the barrier layer in a drift region between agate and a drain, wherein a two dimensional electron gas (2DEG) has anon-uniform lateral distribution in the drift region between the gateand the drain, wherein the stress inducing layer comprises a materialhaving a height that decreases linearly and monotonically in the driftregion in the direction from the gate towards the drain, and wherein the2DEG decreases in density in the drift region between the gate and thedrain.

In another embodiment disclosed herein, a method of fabricating a highelectron mobility field effect transistor (HEMT) comprises forming achannel layer on a substrate, forming a barrier layer on the channellayer, and forming a stress inducing layer on the barrier layer, thestress inducing layer varying the piezo-electric effect in the barrierlayer in a drift region between a gate and a drain, wherein a twodimensional electron gas (2DEG) has a non-uniform lateral distributionin the drift region between the gate and the drain.

These and other features and advantages will become further apparentfrom the detailed description and accompanying figures that follow. Inthe figures and description, numerals indicate the various features,like numerals referring to like features throughout both the drawingsand the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a deposited layer on top of the carrier supplying layerwith varying size openings to induce a stress and modulate the densityof a two dimensional electron gas (2DEG) in a HEMT device in accordancewith the present disclosure;

FIG. 2 shows a deposited layer on top of the carrier supplying layerwith a varying height profile to induce a stress and modulate thedensity of a two dimensional electron gas (2DEG) in a HEMT device inaccordance with the present disclosure;

FIGS. 3A and 3B are flow diagrams for methods of fabricating a HEMTdevice in accordance with the present disclosure;

FIG. 4 shows another embodiment of a stress inducing layer with avarying height profile to induce a stress and modulate the density of atwo dimensional electron gas (2DEG) in a HEMT device in accordance withthe present disclosure; and

FIG. 5 shows a deposited layer on top of the carrier supplying layerwith openings increasing in size from the gate to the drain inaccordance with the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toclearly describe various specific embodiments disclosed herein. Oneskilled in the art, however, will understand that the presently claimedinvention may be practiced without all of the specific details discussedbelow. In other instances, well known features have not been describedso as not to obscure the invention.

Referring now to FIG. 1, a field effect transistor (FET) devicestructure 10 is shown. The FET device structure 10 is composed of astack of III-V layers, such as GaN layer 14 and AlGaN layer 16, grown ona substrate 12 that can be any of the suitable substrates that arecommonly used to grow type III-Nitride materials. Suitable substratesinclude but are not limited to silicon (Si), Sapphire, silicon carbide(SiC), and bulk single crystal gallium nitride (GaN).

The stack of III-V layers may include a buffer layer of GaN or aluminumgallium nitride (AlGaN) grown on the substrate 12. Then a type IIIlayer, such as GaN layer 14, for the channel and carrier travellinglayer, is grown on the buffer layer. Then a type V layer, such as AlGaNlayer 16, for the barrier layer and the carrier supplying layer, isgrown on top of the GaN layer 14. An AlN spacer layer may be between theGaN layer 14 and the AlGaN layer 16 to improve device electricalperformance.

As shown in FIG. 1, on top of the barrier layer layer 16 a stressinducing layer 46 is grown to induce a stress and modulate the charge inthe 2DEG 42, as shown in FIG. 1. In one embodiment the stress inducinglayer 46 is etched with openings 48 in specific areas, as shown in FIG.1, using a mask. The stress inducing layer 46 modulates the stress inthe stress inducing layer 46. The stress varies the piezoelectric effectin the barrier layer 16 and hence the 2DEG 42 in a drift region betweenthe gate 22 and the drain 20.

The lateral control of the stress in the stress inducing layer 46 overthe drift region may be achieved by etching different size openings 48in the stress inducing layer 46 that either monotonically increase ormonotonically decrease in size from the gate 22 to the drain 20depending on the polarity of the stress the deposited layer is exertingon the barrier or charge supplying layer, as shown in FIG. 5 and FIG. 1,respectively. The number of opening is greater than two, as shown inFIG. 1 and FIG. 5.

If it is desired to increase the piezoelectric effect in the driftregion from gate 22 to drain 20, the size of the openings 48 are largernear the gate 22 and decrease in size towards the drain 20, as shown inFIG. 1.

If it is desired to reduce the piezoelectric effect in the drift regionfrom the gate 22 to the drain 20, the size of the openings 48 are madesmaller near the gate 22 and increase in size towards the drain 20.

Since the density of charge in the 2DEG region is determined locally bythe magnitude of the piezoelectric effect, a non-uniform 2DEG 42distribution may be achieved by varying the magnitude of piezoelectriceffect over the drift region as a function of distance from the gate 22towards the drain 20 along the drift region. In the embodiment shown inFIG. 1 the piezoelectric effect in the drift region increases from thegate 22 towards the drain 20, and the size of the openings 48 are largernear the gate 22 and decrease in size towards the drain 20. Such anembodiment results in the 2DEG 42 varying in density such that the 2DEGincreases in density in the drift region from the gate 22 towards thedrain 20.

As described above, if it is desired to reduce the piezoelectric effectin the drift region from gate 22 to drain 20, the size of the openings48 may be made smaller near the gate 22 and increased in size towardsthe drain 20.

In another embodiment, shown in FIG. 2, a stress inducing layer 50 isgrown on top of the AlGaN layer 16. The non-uniform lateral 2DEG 44 isachieved in this embodiment by using Gray scale photolithography to forma stress inducing layer 50 having a lateral profile that has a heightthat varies.

Gray scale photolithography uses a gray scale mask to form a stressinducing layer 50 that has a lateral profile and has a height thatincreases or decreases in the drift region 52 of stress inducing layer50 between the gate 22 and the drain 20, depending on the polarity ofthe stress to be induced.

In the embodiment shown in FIG. 2, the height of the stress inducinglayer 50 increases in the drift region 52 of the stress inducing layer50 starting at the gate 22 and continuing to the drain 20. Further, asshown in FIG. 2 the height of the stress inducing layer 50 may increaselinearly and monotonically in the drift region 52 of the stress inducinglayer 50 starting at the gate 22 and continuing to the drain 20. Thestress inducing layer 50 varies the magnitude of piezoelectric effectover the drift region as a function of distance from the gate 22 towardsthe drain 20 along the drift region. In the embodiment shown in FIG. 2,the stress and the piezoelectric effect in the drift region increasesfrom the gate 22 towards the drain 20. Such an embodiment results in the2DEG 42 varying in density such that the 2DEG 44 increases in density inthe drift region from the gate 22 towards the drain 20. In theembodiment shown in FIG. 4, the height of the stress inducing layer 50decreases in the drift region 52 of the stress inducting layer 50starting at the gate 22 and continuing to the drain 20. Further, asshown in FIG. 4 the height of the stress inducing layer 50 may decreaselinearly and monotonically in the drift region 52 of the stress inducinglayer 50 between the gate 22 and the drain 20. The stress inducing layer50 varies the magnitude of piezoelectric effect over the drift region asa function of distance from the gate 22 towards the drain 20 along thedrift region. In the embodiment shown in FIG. 4, the stress and thepiezoelectric effect in the drift region decreases from the gate 22towards the drain 20. Such an embodiment results in the 2DEG 42 varyingin density such that the 2DEG 44 decreases in density in the driftregion from the gate 22 towards the drain 20.

As described above, if it is desired to reduce the piezoelectric effectin the drift region from the gate 22 to the drain 20, the height of thestress inducing layer 50 decreases in the drift region in the directionfrom the gate 22 towards the drain 20.

The source contact 18 and the drain contact 20 are formed by metalevaporation or metal sputtering to contact the barrier layer 16 afteropening a contact window in the stress inducing layer 46, in theembodiment of FIG. 1, or stress inducing layer 50, in the embodiment ofFIG. 2.

A passivation layer 24 may be grown on top of the stress inducing layer46 or the stress inducing layer 50.

A gate region is then formed by etching a trench through the passivationlayer 24 and the stress inducing layer 46 or 50 in a gate area betweenthe source 18 and drain 20 and into the AlGaN layer 16. In anotherembodiment the etched trench may extend through the AlGaN layer 16 andpartially into the GaN layer 14 to an appropriate depth. A gatedielectric 26 is then deposited over the area between the source 18 andgate 22 and the gate 22 and the drain 20, and also deposited to line theetched trench that extends into the AlGaN layer 16. If the etched trenchextends into the GaN layer 14, then the gate dielectric 26 also linesthe etched trench that extends into the GaN layer 14.

After deposition of the gate dielectric 26, gate metal 22 is formed byevaporation or sputtering and fills the etched trench.

Various alternating passivation and metallization layers may be formedas a part of back-end processing to improve the parasitic resistance ofthe device and provide connection to device pads and/or a package.

Use of the stress inducing layer 46 or 50 to vary the piezo-electriceffect in the barrier or charge supplying layer 16, and hence the 2DEGdensity in the drift region between the gate 22 and the drain 20, mayprovide a significant improvement of the figure of merit (FOM) in typeIII Nitride HEMT devices by achieving a flat electric field distributionin the drift region between the gate 22 and the drain 20. The 2DEG 42 isvaried in the drift region to form a non-uniform lateral 2DEGdistribution 44, as shown in FIGS. 1 and 2. A flat electric fielddistribution results from the non-uniform lateral 2DEG distribution 44along the drift region which provides the improvement in the figure ofmerit (FOM), defined as BV²/Ron.

FIGS. 3A and 3B are flow diagrams for methods of fabricating a HEMTdevice in accordance with the present disclosure.

In step 100 a channel layer is formed on a substrate. Then in step 102 abarrier layer is formed on the channel layer. Next in step 104 a stressinducing layer is formed on the barrier layer, the stress inducing layervarying the piezo-electric effect in the barrier layer in a drift regionbetween a gate and a drain, wherein a two dimensional electron gas(2deg) has a non-uniform lateral distribution in the drift regionbetween the gate and the drain.

In step 106 forming the stress inducing layer on the barrier layerincludes forming a material having a plurality of varying size openingsbetween the gate and the drain, the plurality of varying size openingsdecrease or increase in size from the gate towards the drain.

In step 108 forming the stress inducing layer on the barrier layerincludes forming a material having a height that increases or decreasesfrom the gate towards the drain using gray scale photolithography.

In step 110 a passivation layer is formed over the barrier layer. Instep 112 a gate is formed comprising the steps of forming a trenchextending through the passivation layer and into the barrier layer,depositing a gate dielectric layer in the trench, and forming gate metalon the gate dielectric. In step 114 the forming a trench extendingthrough the passivation layer and into the barrier layer furtherincludes forming the trench to extend into the channel layer.

In step 116 an opening is formed in the stress inducing layer under thedrain for providing contact of the drain to the barrier layer. Andfinally, in step 118 an opening is formed in the stress inducing layerunder a source for providing contact of the source to the barrier layer

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications to the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asdisclosed herein.

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimelement in the singular is not intended to mean “one and only one”unless explicitly so stated. Moreover, no element, component, nor methodor process step in this disclosure is intended to be dedicated to thepublic regardless of whether the element, component, or step isexplicitly recited in the Claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. Sec. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for . .. ” and no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “comprising the step(s) of . . . .”

What is claimed is:
 1. A high electron mobility field effect transistor(HEMT) having a substrate, a channel layer on the substrate and abarrier layer on the channel layer, the HEMT comprising: a stressinducing layer on the barrier layer, the stress inducing layer varying apiezo-electric effect in the barrier layer in a drift region between agate and a drain; wherein a two dimensional electron gas (2DEG) has anon-uniform lateral distribution in the drift region between the gateand the drain; wherein the stress inducing layer comprises a materialhaving a height that decreases linearly and monotonically in the driftregion in the direction from the gate towards the drain; and wherein the2DEG decreases in density in the drift region between the gate and thedrain.
 2. The HEMT of claim 1 further comprising: an opening in thestress inducing layer under the drain for providing contact of the drainto the barrier layer; and an opening in the stress inducing layer undera source for providing contact of the source to the barrier layer. 3.The HEMT of claim 1 further comprising: a passivation layer over thestress inducing layer.
 4. The HEMT of claim 1 wherein: the substratecomprises silicon (Si), sapphire, silicon carbide (SiC), OR bulk singlecrystal gallium nitride (GaN); the channel layer comprises a GaN layer;and the barrier layer comprises a AlGaN layer.
 5. The HEMT of claim 3further comprising: a trench extending through the passivation layer andinto the barrier layer; and a gate dielectric layer in the trench; andgate metal on the gate dielectric.
 6. The HEMT of claim 5 wherein thetrench extends into the channel layer.